Mechanical crushers operate by crushing, or breaking, large particles between two surfaces. Specific types of mechanical crushers include jaw crushers, roll crushers, gyratory crushers, and cone crushers. All crushers require a substantial input of power because so much work is required to crush rock or similar materials. All form of crushers also require some sort of feed mechanism, such as a conveyor belt and feed hoppers, to provide a continuous supply of raw material to the crusher.
Jaw crushers comprise a movable jaw and a stationary jaw. The movable jaw is driven toward the stationary jaw with great force. The jaws thus function like a gigantic pair of pliers - material to be crushed is squeezed between the movable jaw and the stationary jaw. A variety of force multiplying mechanisms are employed in different jaw crushers to convert the mechanical input energy, typically from an electric motor, into the large linear forces required by the movable jaw. Jaw crushers are relatively simple and powerful. Because a portion of their operating cycle is necessarily devoted to opening the jaws, they can only operate intermittently.
Roll crushers operate by squeezing the material between two rollers. Their operating principle is similar to that of a jaw crusher, but each jaw is replaced by a driven roller. Their primary advantage vis-a-vis jaw crushers is that, since they have no opening stroke, they can operate continuously. Both jaw and roller crushers are oriented with their crushing opening vertical, so that material to be crushed is fed into the crusher by gravity.
Gyratory or cone crushers are both classified as rotary crushers. Both gyratory and cone crushers operate by continuously rotating an inner conical member, the mantle, eccentrically relative to an outer stationary conical member, the liner. The main difference between the two is that, in a gyratory crusher, the space between the two members is essentially vertical, whereas in a cone crusher, the space between the two members is inclined more toward the horizontal. The distinction between the two is illustrated in FIGS. 1 and 2, showing a plan view and a cross section through a cone crusher, respectively, and FIG. 3, showing a cross section through a gyratory crusher. In both types of rotary crushers, gyratory and cone, the slope of the mantle and slope of the liner are different, so that the space between the mantle and liner decreases as material moves downward between them. Material to be crushed falls downward, under the influence of gravity, between the stationary and moving members. In the case of a cone crusher, centrifugal force may have a secondary effect in moving the material to be crushed. Large material falling through this space is wedged between the mantle and the liner, and the material is crushed as it falls downward. When it is reduced to a small enough size, it passes out of the crusher through the smallest gap between the jaws, which is the "closed side setting". Crushers usually receive an input of rock, or similar material, in a wide range of sizes. Material which is small enough should pass through the crusher without being further crushed. Material which is large enough to be acted upon by the crushing apparatus will be crushed small enough to exit via the output aperture. However, there are a number of reasons why crushing does not proceed as simply or smoothly as this basic description would suggest.
The major technical characteristics desired of a crusher are:
High throughput, PA1 uniform output product, having a specific size distribution, and large size reduction on a single pass.
Each of these technical characteristics result in a better product and/or a lower cost of producing the product. Each technical characteristic is discussed below:
Throughput of a crusher is usually measured in mass per unit time, e.g. tons/hour, and is determined by such factors as: initial size of the material to be crushed, required size reduction, and the material's resistance to crushing. When one observes a rock crusher in action, what is seen is a great deal of churning about as the mass of particles attempt to work through the crusher. Small particles and large particles interfere with each other--resulting in a great deal of congestion. The net result is that the throughput is often considerably less than expected. The actual throughput is a complex function of the rock's hardness, shape (does it break into angular fragments, long "platy" fragments, or into rounded fragments?), and its size distribution (does much of the total mass consist of small particles, or large particles, or is the mass somewhat uniformly distributed over all sizes?). Size distribution is important because small particles cause "bridging" or "packing" effects which act to slow the flow of large particles through the crusher.
An output product of uniform size is almost always preferred. Even in those cases where a specific broad size distribution is required (e.g. for asphalt fillers), separate size fractions may be produced separately, and then combined. This is done because it is so difficult to adjust crushing machines to produce the desired size distribution.
In practice it is found that achieving a uniform product may require passage through more than one crusher--e.g. the output of a coarse crusher serves as the input to a finer crusher. Hence one desired characteristic of a crusher is that it be able to produce a uniform size of output by itself, with no additional equipment being required.
It is also found that the crushing operation may produce an excessive amount of undesired "fines", for example material able to pass through a 100 mesh screen. Fines often have to be classified as waste, and thus discarded. Excessive production of fines can thus increase the cost of the output product.
A large amount of size reduction on a single pass is also desirable. I.e. large input particles should be reduced to small particles in a single pass.
These desired characteristics serve to minimize the amount of equipment required to perform the overall crushing operation, and thus to minimize equipment and processing costs.
In practice, a crusher will often be specified based on previous engineering experience on similar jobs, and the crusher is found to be incorrectly sized when installed. Frequently, it will be found to have inadequate capacity. More rarely it will have excess capacity. An inadequate crusher can become a bottleneck, causing an entire processing plant or mill to become uneconomic, and even causing a project to fail.
The cure for an inadequate crusher usually involves installation of a second crusher in parallel with the first. This in turn leads to delays, additional expense, and re-engineering of the process, all of which contribute to avoidable economic costs.
The industry has a long felt need for a crusher having high throughput, uniform output, and substantial size reduction on a single pass. It should do this without significant increases in size, cost, or power required.